Electronic switch for converting a pulse signal into an analog signal

ABSTRACT

An electronic switch for converting a pulse signal into a continuous analog signal by connecting a d.c. voltage to an integrating circuit under the control of pulses of the pulse signal comprising two switching elements connected by a voltage divider, and lying between the poles of the d.c. voltage source, the divider ratio being selected to compensate the effects on the analog output signal at the output of the integrating circuit of temperature dependence of the switching elements.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 742,686,filed Nov. 17th, 1976.

BACKGROUND OF THE INVENTION

The invention relates to a largely temperature stable electronic switchfor use as the input member of a circuit for converting a pulse signalinto a continuous analog signal having an amplitude representative ofthe duty cycle of the pulse signal. For the purpose of conversion, pulsesignals are supplied to an electronic switch, the pulses having a pulseduration τ and a period duration T. The electronic switch connects ad.c. voltage U to an integrating element, controlled by these pulses, atthe output of which element an analog signal U_(M) with the size

    U.sub.M =(τ/T)·U

is obtained.

For certain applications, e.g. for the purpose of producing a tuningd.c. voltage for electronic tuners in television devices, the analogsignal U_(M) must be varied in relatively small steps; in this examplethe steps are 1/10 000th of the d.c. voltage U. As a result ofintegrated circuit technology, the minimum pulse duration τ is, forexample, 5 μsec. For stability of the analog signal U_(M) in relation tothe temperature demands ##EQU1## are made within an operatingtemperature range of 20° . . . 50° C. so that the pulse duration τ maynot vary within the stated temperature range more than 1/1000th τcorresponding to Δτ=5 nsec. This makes extremely high demands ontemperature stability of the electronic switch.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a circuit for an electronicswitch which overcomes the dependence on temperature of known electronicswitches as extensively as possible.

According to the invention, there is provided an electronic switch foruse in a circuit for converting a pulse signal into a continuous analogsignal by connecting a d.c. voltage to an integrating circuit under thecontrol of pulses of the pulse signal, said switch comprising a firstswitching element with a first switchable path, a voltage dividerconnected by one end tc said first switchable path, a second switchingelement with a second switchable path connected to the other end of saidvoltage divider with said switchable paths and said voltage dividerlying between the poles of the d.c. voltage and with the divider ratioof said voltage divider compensating the effects on the analog outputsignal at the output of the integrating circuit, connected to saidvoltage divider, of temperature dependence of said first and secondswitching elements.

Further according to the invention, there is provided an electronicswitch for the same purpose, which connects a d.c. voltage to anintegrating element in time with the sequence frequency of a pulsesignal, characterized in that the operable path of a first switchingelement, a voltage divider and the operable path of a second switchingelement lie between the poles of the d.c. voltage source; and that thedivider ratio of the voltage divider is selected such that the effectsof the temperature dependence of the first and second switching elementon the analog signal at the output of the integrating element connectedto the voltage divider is compensated.

Still further according to the invention, there is provided a circuitfor converting a pulse signal into an analog signal comprising a d.c.voltage source, an electronic switch, a switching path for saidelectronic switch, connected between the poles of said d.c. voltagesource, a first switching element for said electronic switch, a secondswitching element for said electronic switch and a voltage dividerconnecting said first and second switching elements, an integratingcircuit connected to said electronic switch to provide an analog outputsignal with the temperature dependence of said first and secondswitching elements compensated for by the said voltage divider.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail, by way ofexample, with reference to the drawings in which:

FIG. 1 is a graphical representation of the course of various signals inthe circuit of FIG. 2.

FIG. 2 is a circuit diagram of an embodiment of an electronic switchaccording to the invention.

FIG. 3 is a further graphical representation of signals in the circuitof the invention, and

FIG. 4 is a circuit diagram of a specific tested circuit in accordancewith the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

At line (a), FIG. 1 shows a clock pulse signal with the clock frequencyof 200 kHz, for example, for a control circuit connected ahead of theinput member of an electronic switch, a pulse signal controlled by thosepulses is shown for a discrete analog voltage U_(M) at line (b). Thispulse signal lies at the input of the electronic switch, at point A inFIG. 2. Line (c) of FIG. 1 shows the output signal of the electronicswitch in outline at point B. The mode of operation of the electronicswitch will be described in greater detail with the aid of FIG. 2.

The input signal to the electronic switch (FIG. 1, line b) is applied atthe point A at the levels "Low"=0 V and "High"=+4 V. The "High" pulsebecomes effective via R₁ C₁ (RC element) at the base of the transistorT₁ which is of the NPN type, it controls this transistor into saturationso that its collector potential almost equals the emitterpotential--thus 0 V. The flank of the input pulse becomes completelyeffective at the base of the transistor T₁ via C₁, whereby thetransistor is unblocked very rapidly and is held in the unblockedposition by the current which is determined by the resistor R₁.

The "High" pulse lying at point A cannot reach the transistor T₂ of thePNP type because the base of the previously connected transistor T_(2a)of the NPN type is fixed at a d.c. voltage potential by the blocked offvoltage divider 10, which potential corresponds approximately to halfthe amplitude of the "High" pulse (in the example +2 V). The transistorT₂ remains open, i.e. isolating point B from the d.c. voltage source U,which is blocked off by C₁₁, if there is no current flowing to the pointB of the circuit.

A "Low" signal at point A becomes effective at the base of thetransistor T₁ very rapidly via R₁ C₁ and clears away its charge wherebyT₁ is non-conductive and its collector accepts the potential at point B.The "Low" signal controls the transistor T_(2a) via R₂ C₂ and thuscontrols transistor T₂ in the flow direction whereby the collector ofthe transistor T₂ is, in practice, brought to the emitter potential,thus to the voltage +U. The flank of the input pulse becomes completelyeffective via C₂ at the emitter of the transistor T_(2a) and thus alsoat the base of the transistor T₂, whereby the transistor T₂ is veryrapidly unblocked and is held in the unblocked state by the currentwhich is determined by the resistor R₂.

A "High" signal following a "Low" signal becomes effective, asdescribed, at transistor T₁, i.e. the signal is made steeper by C₁. This"High" signal in fact blocks transistor T_(2a), also made steeper viaC₂, into the currentless state, yet not transistor T₂ whose base chargemay only flow away via the resistor R₃ whereby the storage time of thetransistor T₂ becomes effective. The voltage curves shown in FIG. 3 areproduced at the collectors of the transistors T₁ and T₂ (solid curves).Switching from 0 volts up to the voltage +U takes place at thecollectors of the transistors T₁ and T₂ at almost the same steepnessduring the time t_(r). While disconnection of +U to 0 V takes place atthe collector T₁ at t_(f) ≈t_(r), the storage time of the transistor T₂becomes effective at the collector and disconnection takes place onlyafter the time t_(s).

As described at the outset, the transistor T₁ is switched from "High" to"Low" during exchange of the signal at point A, while, the transistor T₂is unblocked. When there is a change in the surrounding temperature forthis switching condition it is observed that, with increasing heating ofthe transistors, the steepness of the turn on flank increases and thesteepness of the turn off flank decreases (shown in broken lines). Whenthere is a change in the signal at point A from "Low" to "High" then itis evident, with an increase in the surrounding temperature, that thesteepness of the turn on flank increases and the storage time of theturn off flank substantially increases (indicated in broken lines).

If the collectors of the transistors are not directly connected but areconnected via a voltage divider comprising the resistors R₄ and R₅ andif the integrating element 11 is connected to the voltage divider pointB, then with a suitable choice of the resistance value, the temperaturedependence of the electronic switch may be eliminated as indicated inthe last line of FIG. 3 (signal at point B) (broken line signal curve atincreased temperature).

With the tested circuit of FIG. 4, in which a further transistor T₃having an emitter resistor R₆ is connected up which holds the currentload of the d.c. voltage source U extensively constant independent ofthe analog value selected, the change in the voltage U_(M) of the analogsignal at the output of the integrating element is no more than1/3000=0.033% referring to U_(M) when there is a surrounding temperaturebetween 20° C. and 50° C. Thus the clock pulse was chosen at 200 kHz,the pulses supplied to the electronic switch were chosen at a pulseduration of τ=5 μsec and the d.c. voltage U was chosen at 33 V. Thewidth of the step of the analog signal ΔU_(M) was 3.3 mV.

When testing the circuit in accordance with the invention a furtheradvantage became evident i.e. that by incorporating the storage timet_(s) into the analog value, the frequency dependence brought about bythe propagation time in the switch is reduced. A change in the clockpulse frequency by ±10% merely causes a change in the analog voltagevalue by ±0.05% at the output of the integrating element.

It will be understood that the above description of the presentinvention is susceptible to various modification changes andadaptations.

What is claimed is:
 1. An electronic switch for use in a circuit forconverting a pulse signal into a continuous analog signal by connectinga d.c. voltage to an integrating circuit under the control of pulses ofthe pulse signal, said switch comprising a first switching transistorwith a first switchable path, a voltage divider connected by one end tosaid first switchable path, and a second switching transistor ofopposite semiconductor type to said first transistor, with a secondswitchable path connected to the other end of said voltage divider eachsaid path having a temperature-dependent switching characteristic, withsaid switchable paths and said voltage divider lying between the polesof the d.c. voltage, and wherein said voltage divider includes at leasttwo impedances each connected between a respective switching transistorand a point of said divider intermediate its ends, providing the inputfor the integrating circuit, with the values of said impedances beingselected for giving said divider a divider ratio, with respect to saidpoint intermediate its ends, for causing the volt-second narrowing ofthe pulse output on the collector of one said transistor with increasingtemperature to be at least partly offset by the correspondingvolt-second widening of the pulse output on the collector of the othersaid transistor at said point intermediate the ends of said divider,thereby to effect compensation of the effects on the analog outputsignal at the output of the integrating circuit, connected to saidvoltage divider, of temperature dependence of the switchingcharacteristics of said paths of said first and second switchingtransistors.
 2. A circuit including an integrating element and anelectronic switch for connecting a d.c. voltage to the integratingelement under control of a pulse signal, in order to convert the pulsesignal into a continuous analog signal at the output of the integratingelement, said switch comprising first and second switching transistorsof mutually complementary semiconductor types, each presenting aswitchable current path having a temperature-dependent switchingcharacteristic; a voltage divider connected between said first andsecond transistors to form a series circuit with said current paths ofsaid first and second transistors; and a source of such d.c. voltagehaving two poles each connected to a respective end of said seriescircuit, and said integrating element having its input connected to saidvoltage divider at a point intermediate its ends; and wherein saidvoltage divider includes at least two impedances each connected betweena respective switching transistor and the input of said integratingelement, with the values of said impedances being selected for givingsaid divider a divider ratio, with respect to said point intermediateits ends, for causing the volt-second narrowing of the pulse output onthe collector of one said transistor with increasing temperature to beat least partly offset, at said integrating element input, by thecorresponding volt-second widening of the pulse output on the collectorof the other said transistor, thereby to effect compensation for thetemperature dependence of the switching characteristics of said currentpaths of said switching transistors at the output of said integratingelement.
 3. An arrangement as defined in claim 2, wherein said firstswitching transistor is connected to be rendered conductive by a highpulse signal value and said second switching transistor is connected tobe rendered conductive by a low pulse signal value.
 4. An arrangement asdefined in claim 2, and comprising a third transistor connected forkeeping the current loading of said d.c. voltage source extensivelyconstant independent of variations in the analog value.
 5. Anarrangement as defined in claim 2, comprising a further transistorconnected ahead of one of said switching transistors for isolating saidone switching element from pulse signal portions which render the othersaid switching transistor conductive.
 6. An arrangement as defined inclaim 4 wherein said third transistor has its collector-emitter pathconnected between said poles of said d.c. voltage source through anemitter resistor, and has its base connected to the input of saidelectronic switch.
 7. An arrangement as defined in claim 5 comprising ablocked off voltage divider connected for keeping the base of saidfurther transistor at a predetermined d.c. voltage potential.